This invention generally relates to rotary vane motors, and is specifically concerned with a two-stage rotary vane motor for effectively extracting mechanical energy from a variable flow of an expanding cryogenic gas.
Rotary vane motors are well known in the prior art. Such motors (sometimes known as "expanders") typically comprise a housing having a cylindrical interior, and a rotor eccentrically mounted therein. The rotor includes a cylindrically shaped body having a plurality of uniformly spaced, radially oriented slots for slidably receiving a plurality of rectangularly shaped vanes. Both the housing and the rotor body within the cylindrical enclosure defined by the housing leaves a gap between the rotor and the housing that is crescent-shaped in cross section. In operation, pressurized drive fluid (usually compressed air) is admitted in an inlet port in the housing located at one of the narrow ends of the crescent-shaped gap. The pressurized fluid pushes against the trailing faces of the slidable vanes, thereby rotating the rotor body. Centrifugal force radially slings the vanes out of their slots such that their outer edges sealingly engage the surface of the cylindrical enclosure. The vanes reciprocate in their respective slots as their outer edges sealingly and slidably engage the interior surface defining the cylindrical enclosure. The pressurized fluid is expelled out an outlet port located at the other end of the crescent-shaped gap in order to create the pressure differential necessary to drive the rotor assembly.
Such prior art rotary vane motors are well adapted for powering tools such as pneumatic wrenches and grinders where the operating speeds of the motor shaft are greater than 2000 rpm, and where a steady mass flow rate of pressurized drive fluid in the form of a supply of compressed and lubricant-containing air is consistently supplied by the shop air compressor. The applicants have observed, however, that such prior art rotary vane motor designs are not well suited for use at relatively low rotational speeds (i.e., under 1500 rpms) where the mass flow rate of the drive fluid substantially varies. Such an application for a low speed rotary vane motor may occur, for example, in a cryogenic refrigeration system powered by a tank of liquefied carbon dioxide such as that disclosed in co-pending U.S. Ser. No. 08/501,372 filed Jul. 12, 1995, also assigned to the Thermo King Corporation of Minneapolis, Minn. In such an application, the rotary vane motor is used to drive an evaporator blower and an alternator to recharge the battery that powers the refrigeration control system, and low rotational speeds are preferred to enhance the efficiency of the evaporator blower.
At low rotational speeds, in order for the rotary vane motor to efficiently convert the energy of the expanding gas into rotary energy, the components which comprise the rotor assembly must be properly sized. If the overall mass flow rate of the expanding cryogen remained constant during the operation of such refrigeration systems, proper sizing of the rotor assembly components would not be a critical issue. However, the applicants have observed that the mass flow of the cryogen gas used as drive fluid can begin at 350 pounds per hour during the "pull-down" portion of the refrigeration cycle, but then level off to a rate of only 100 pounds per hour as the set point temperature for the system is approached. Presently, there is no known rotary vane motor that can efficiently convert energy from the expanding cryogen gas into rotary energy at slow rotational speeds and over such a broad range of cryogen mass flow rates. If the motor is large enough to efficiently convert such energy at a mass flow rate of 350 pounds per hour, then it will be grossly oversized for any such efficiency at a mass flow rate of 100 pounds per hour. On the other hand, if the motor is small enough for efficient operation at 100 pounds per hour, then the rpms will be too high when the mass flow rate increases to 350 pounds per hour.
The foregoing illustrates limitations known to exist in prior art rotary vane motors and methods. Thus it is apparent that it would be advantageous to provide a rotary vane motor that overcomes the limitations illustrated in the prior art. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.